Corrosion has long been a problem when certain metals or alloys are used in applications in which they come into contact with an aqueous medium. For example, in heat-transfer systems, such as those found in internal combustion engines, alcohol-based heat transfer fluids (i.e., antifreezes can be very corrosive to the metal surfaces of the heat-transfer systems. Compounding this problem is that the corrosion is accelerated under normal engine operating conditions (i.e., high temperatures and pressures).
Aluminum surfaces, are particularly susceptible to corrosion. See Darden et al., "Monobasic/Diacid Combination as Corrosion Inhibitors in Antifreeze Formulations," Worldwide Trends in Engine Coolants, Cooling System Materials and Testing, SAE Int'l SP-811, Paper #900804, pp. 135-51 (1990)("SAE SP-811").
Indeed, aluminum surfaces are susceptible to several types of corrosion including general corrosion, pitting and crevice corrosion as well as cavitation-erosion corrosion. These types of corrosion, however, typically occur under different conditions and thus, affect different types of aluminum surfaces. For example, general corrosion usually occurs on aluminum surfaces which are readily susceptible to corrosion because they are poorly inhibited or because they are, subject to "heat-rejecting" conditions (e.g., cylinder heads and engine blocks) or "heat-accepting" conditions (e.g., radiators and heater cores).
Pitting/crevice corrosion usually occurs on the thin aluminum sheets used in radiators or heater cores. Such corrosion generally results from localized penetration of the oxide film which would otherwise cover and protect the aluminum surfaces. See SAE SP-811.
Cavitation-erosion corrosion ("CE-type" corrosion), like pitting/crevice corrosion, attacks the protective oxide film but results from explosion of bubbles on the aluminum surfaces. See SAE SP-811 at 136 CE-type corrosion can be accelerated by the formation of foam in the cooling system. Foam results from air bubbles which are entrapped and agitated in the cooling system. See, e.g., Nalco, "Cooling System Liner/Water Pump Pitting," Technifax TF-159 (1988). Thus, water pumps, which are used to circulate antifreeze coolants throughout a vehicle's cooling and/or heating systems, are particularly susceptible to CE type corrosion. This is because bubbles are readily formed on the trailing sides of the water pump impeller blades due to locally reduced pressure and consequent boiling caused by the high rotation rate. When these bubbles collapse in higher pressure areas in the water pump, they can erode the metal in these areas. This process can eventually destroy the impeller causing loss of pumping performance and/or can perforate the pump body leading to loss of engine coolant. See, e.g., B. D. Oakes, "Observation on Aluminum Water Pump Cavitation Tests," Second Symposium on Engine Coolants, ASTM STP 887, pp. 231-48 (1986).
The corrosion of aluminum surfaces has become a significant concern in the automotive industry because of the increasing use of such lightweight materials. See, e.g., Ward's Auto World, p. 22 September, 1996); Ward's 1996 Automotive Yearbook, p. 27 (58th ed. 1996). For example, heat exchangers in cars and light duty trucks are now being constructed using aluminum components including the water pumps. See Hudgens et al., "Test Methods for the Development of Supplemental Additives for Heavy-Duty Diesel Engine Coolants," Engine Coolant Testing: Second Volume, ASTM STP 887, R. E. Beal, Ed., ASTM, Philadelphia, 1986, pp. 189-215; Oakes, B. D., "Observations on Aluminum Water Pump Cavitation Tests," Engine Coolant Testing: Second Volume, ASTM STP 887, R. E. Beal, Ed., ASTM, Philadelphia, 1986, pp. 231-248; Beynon et al., "Cooling System Corrosion in Relation to Design and Materials," Engine Coolant Testing: State of the Art, ASTM STP 705, W. H. Ailor, Ed., ASTM, Philadelphia, 1980, pp. 310-326. In particular, CE-type corrosion has become a significant concern because, aside from mechanical seal failures caused by high thermal stresses and inadequate lubrication, CE-type corrosion is one of the leading causes of water pump failures. See, e.g., E. Beynon, supra at 310-326 (1980).
In general, corrosion inhibitors have been used to protect the metal surfaces used in heat transfer systems. For example, triazoles, thiazoles, borates, silicates, phosphates, benzoates, nitrates, nitrites and molybdates have been used in antifreeze formulations. See, e.g., U.S. Pat. No. 4,873,011; see also, SAE SP-811 at pp. 135-138, 145-46. However, such corrosion inhibitors have several problems, including expense, and inadequate long-term protection. See U.S. Pat. No. 4,946,616, col. 1, lines 31-45; U.S. Pat. No. 4,588,513, col. 1, lines 55-64; SAE SP-811, pp. 137-38. Accordingly, automobile manufacturers have begun using, and several now require, organic acid based (or extended life) corrosion inhibitors such as mono- and/or di-carboxylic acids. A number of carboxylic acid corrosion inhibitors have been described. See, e.g., U.S. Pat. Nos. 4,382,008, 4,448,702 and 4,946,616; see also, U.S. Pat. application Ser. No. 08/567,639, incorporated herein by reference.
However, carboxylic acid corrosion inhibitors, while effective at protecting against general and pitting/crevice types of aluminum corrosion, are generally ineffective as CE-type corrosion inhibitors. See, e.g. D. E. Turcotte, "Engine Coolant Technology, Performance and Life for Light Duty Application," Fourth Symposium on Engine Coolants (1997). Indeed, many of the known aluminum corrosion inhibitors, while effective at protecting against one or more types of aluminum corrosion, are generally not known to be effective at inhibiting all types of aluminum corrosion. For example, silicates and phosphate salts known to be effective at inhibiting general corrosion and CE-type corrosion, are not known to inhibit polymerizable/crevice corrosion. Also, nitrates which are known to be effective pitting/crevice or CE-type corrosion inhibitors, are not known to inhibit general or CE-type corrosion. Similarly, polymeriable-acid graft copolymers have been used in traditional antifreeze formulations (i.e., containing silicates, phosphates and/or borates) as general corrosion inhibitors for the protection of heat-rejecting aluminum surfaces. See U.S. Pat. Nos. 4,392,972 and 4,904,114. However, such grafted copolymers are not known to be effective as pitting/crevice or CE-type corrosion inhibitors.
Other polymeric compound have been suggested as CE-type corrosion inhibitors for aluminum and aluminum alloys. For example, U.S. Pat. No. 5,288,419 discloses the use of a certain class of polymeric polycarboxylates as CE-type corrosion inhibitors. However, it has been shown that such polymeric polycarboxylates do not consistently pass ASTM standards for CE-type corrosion inhibitors.
Moreover, while other CE-type corrosion inhibitors are known, many of these corrosion inhibitors are metal-specific for non-aluminum surfaces, or are undesirable or unacceptable for the organic acid based antifreeze formulations used today. For example, silicates and phosphate salts, known CE-type corrosion inhibitors of aluminum surfaces, are unacceptable because they have been prohibited for use in organic acid formulations by a number of original equipment automotive manufacturers. See, e.g., General Motors Engineering Standards, "Long-Life Automotive Engine Coolant Antifreeze Concentrate-Ethylene Glycol," Specification No. GM 6277M; Ford Engineering Material Specifications, "Coolant, Organic Additive Technology, Concentrate," Specification No. WSS-M97B44-C; Chrysler Corporation Engineering Standards, "Engine Coolant-Glycol Type-Inhibited-Production and Service Use," Standard No. MS-9769. See also, U.S. Pat. No. 4,146,488 (discloses the use of blends of monoethanolamine borate and graft copolymer salts as effective in providing cast iron corrosion protection).
Thus, there remains a need for an effective and reliable method of inhibiting CE-type corrosion of aluminum surface which is compatible with the use of carboxylic acid based antifreeze formulations.